Completed overall of BalanceChange and Transaction.  Main change involved BalanceChange keeping track of Credits and Debits, which moved this responsibility from Transaction.  Amounts returns null if Credits and Debits are not alligned to maintain the invariant of Amounts summing to zero.  Also removed Payment since all of its functionality is taken care of by BalanceChange.  Payment for a transaction and payment between participants is now taken care of by getDebitsTotal(), getParticipants(), setCredit(), getDebitCreditDiff() and isPaymentCOmplete() provided by balance changed.
Merge branch 'master' of https://github.com/dcashman/BillSplit